class Account(object):
    def __init__(self,rate):
        self.__amt=0
        self.rate=rate
        
    @property
    def amount(self):
        return self.__amt
    
    @property
    def cny(self):
        return self.__amt*self.rate
    
    @amount.setter
    def amount(self,value):
        if value<0:
            print("Sorry, no negetive amount in the account")
            return 
        self.__amt = value
        
if __name__ =='__main__':
    acc = Account(rate=6.6)
    acc.amount=20
    print("Dollar amount:",acc.amount)
    print("In CNY:",acc.cny)
    acc.amount = -100
    print("Dollar amount:",acc.amount)